1. Field of the Invention
The present invention relates to an ultrahigh-purity ferroelectric thin film, more particularly, to an ultrahigh-purity ferroelectric thin film that can be used for infrared sensor, piezoelectric filter, vibrator, laser modulator, optical shutter, capacitor film, nonvolatile memory, etc.
2. Description of the Prior Art
Ferroelectric thin films need heat treatment to increase crystallinity in the final stage of their forming process so that they have good characteristic properties, no matter what the process is. This conventional practice poses a problem because increased crystallinity is accompanied by the growth of crystal grains which leads to an increased leakage current that flows through grain boundaries. This problem prevents ferroelectric thin films from being improved further in their characteristic properties.
The foregoing holds true of ferroelectric thin film of Pb-containing perovskite type. It also needs heat treatment to increase crystallinity for its enhanced ferroelectricity. As crystal grains grow during heat treatment, impurities separate out at the grain boundaries to permit more leakage current to flow through them. It is conceivable that leakage current will decrease if impurities are removed. However, this idea is not practicable because it is difficult to purify Pb, La, Ti, Zr, etc. which constitute the ferroelectric thin film. In fact, it is very difficult to lower the content of their impurities below 0.1 ppm. Incidentally, as far as Pb-containing ferroelectric thin film is concerned, no one has reported that its characteristic properties are improved by purification.